The present invention relates to an arrangement for limiting the terminal voltage in an alternator. The alternator has a fixed stator winding acting on rectifiers, an excitation winding which rotates with the alternator rotor, a controller and a first slip ring contact which load the excitation winding with excitation current as a function of the magnitude of a delivered voltage. The alternator has at least one free-wheeling valve which is conductive during a controller-side interruption of the excitation current and is connected in parallel with the excitation winding.
It is known that only alternators with integrated rectifier stacks are used in motor vehicles. Electronic controllers for influencing the excitation current are frequently also an accompanying integrated alternator component.
According to FIG. 1, which shows a conventional arrangement, the stator winding 1, usually provided with star connection, acts on the negative pole B- of the vehicle battery (vehicle ground) via a first half and on the positive pole B+ of the battery via a second half of a 6-valve three-phase rectifier bridge 2. There is also a further bridge half 3 via which the stator winding acts, also with positive poling in relation to the vehicle ground, on a separate adjacent terminal D+. A controller 4 receives its operating power from the terminal D+. The controller 4 supplies the rotating excitation coil 5 with current in accordance with the voltage delivered at the terminal D+.
It is known that this robust configuration offers a series of advantages in comparison with dynamos, namely small size and weight, in particular with respect to power delivery, good charging current yield even at a low engine speed, easy radio shielding, low maintenance requirements and a long service life.
There is an inherent disadvantage of such dynamos, however, which arises in the case of sudden load dumping, for example the accidental disconnection of the battery terminal when the engine is running or the presence of a loose contact in the main current path. This disadvantage is that, for a brief time, a high voltage is delivered into the vehicle electrical system which can irreparably destroy electronic equipment subjected to this voltage or electronically driven loads (so-called "load-dump situation").
This damaging overvoltage of limited duration comes about when a current control element 6 included in the controller 4 is actually inhibited as a result of a voltage rise sensed simultaneously at the adjacent terminal D+ by a voltage registering arrangement 7. Thus, in a manner which per se reduces terminal voltages, the controller 4 no longer feeds excitation current into the excitation winding 5. The excitation current which flowed immediately before through the excitation winding 5 cannot drop suddenly to zero but instead is maintained for a period via a free-wheeling valve 8 required for reasons of insulation protection, or decays only gradually. To this extent, there is a delay of the reduction of the magnetic excitation and thus an undesired support and prolongation of the overvoltage pulse. This overvoltage pulse can thus last, for example, for 300 ms and at the same time reach up to 200 volts with an equivalent internal resistance of only 2 ohms.
It is therefore an object of the present invention to provide a device for limiting the terminal voltage in an alternator, which ensures with simple means the protection of connected loads from overvoltage surges due to load dumping.
This and other objects are achieved by the present invention which provides an arrangement for limiting the terminal voltage in an alternator having a fixed stator winding acting on rectifiers, an excitation winding which rotates with the alternator rotor, a controller and a first slip ring contact which load the excitation winding with excitation current as a function of the magnitude of a delivered voltage, and at least one free-wheeling valve which is conductive during a controller-side interruption of the excitation current and is connected in parallel with the excitation winding. The arrangement comprises an auxiliary winding on the rotor, this auxiliary winding continuing the excitation winding with the same direction of winding, with the auxiliary winding and the excitation winding being connected at a first connection point. A second slip ring contact operatively connects the first connection point to a machine terminal supplying excitation current. A zener diode is connected in series with the auxiliary winding, and a third slip ring contact operatively connects the auxiliary winding to a ground terminal of the alternator or of the controller.
The device according to an embodiment of the present invention in an alternator permits a rapid field degradation by defined and rapid conversion of the magnetizing energy of the excitation circuit into heat. At the same time, the overvoltage peak is utilized to deliver a current for field compensation of the excitation winding with the effect of reducing the induced voltage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.